In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes modes such as PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), FFS (fringe field switching) and FPA (field-induced photo-reactive alignment). A classification based on a driving mode in the device includes PM (passive matrix) and AM (active matrix). The PM is classified into static, multiplex and so forth, and the AM is classified into TFT (thin film transistor), MIM (metal-insulator-metal) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type depending on the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. This composition has suitable characteristics. An AM device having good characteristics can be obtained by improving the characteristics of this composition. Table 1 below summarizes the relationship between these two characteristics. The characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase is approximately −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Response time that is one millisecond shorter than that of the other devices is desirable. Thus a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable. The elastic constant of the composition relates to the contrast ratio of the device. A large elastic constant of the composition is desirable for increasing the contrast ratio of the device.
TABLE 1Characteristics of Compositions and AM DevicesNo.Characteristics of CompositionsCharacteristics of AM Devices1a wide temperature range of aa wide temperature range innematic phasewhich the device can be used2a small viscositya short response time3a suitable optical anisotropya large contrast ratio4a large positive or negativea low threshold voltage and lowdielectric anisotropypower consumption,a large contrast ratio5a large specific resistancea large voltage holding ratioand a large contrast ratio6a high stability to ultravioleta long service lifelight and heat7a large elastic constanta large contrast ratio and ashort response time
The optical anisotropy of the composition relates to the contrast ratio of the device. A large optical anisotropy or a small optical anisotropy, namely a suitable optical anisotropy, is necessary depending on the mode of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of operating mode. A suitable value is approximately 0.45 micrometers for a device having a mode such as TN. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small power consumption and a large contrast ratio of the device. A large dielectric anisotropy is thus desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. It is thus desirable that a composition should have a large specific resistance at a temperature close to the maximum temperature of a nematic phase as well as at room temperature in the initial stages. It is desirable that a composition should have a large specific resistance at a temperature close to the maximum temperature of a nematic phase as well as at room temperature, after it has been used for a long time. The stability of the composition to ultraviolet light or heat relates to the service life of the device. The device has a long service life when the stability is high. Characteristics of this kind are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
In a conventional liquid crystal display device, the homeotropic alignment of liquid crystal molecules is achieved by a specific polyimide alignment film. The effect of a polymer is utilized for a liquid crystal display device with a polymer sustained alignment (PSA) type. First, a composition to which a small amount of a polymerizable compound has been added is poured into a device. Next, the composition is irradiated with ultraviolet light, while a voltage is applied between the substrates of this device. The polymerizable compound is polymerized to give a network structure of a polymer in the composition. In this composition, the polymer makes it possible to adjust the alignment of liquid crystal molecules, and thus the response time of the device is decreased and image burn-in is improved. This kind of effect of the polymer can be expected for a device having a mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
In contrast, a liquid crystal composition including a polymer and a polar compound is used for a liquid crystal display device without alignment films. First, a composition to which a small amount of a polymerizable compound and a small amount of a polar compound have been added is poured into a device, where the liquid crystal molecules are aligned by the action of the polar compound. Next, the composition is irradiated with ultraviolet light, where the polymerizable compound is polymerized, stabilizing the alignment of the liquid crystal molecules. In this composition, the alignment of the liquid crystal molecules can be adjusted by the polymer and the polar compound, and thus the response time of the device is decreased and image burn-in is improved. Furthermore, a step for forming an alignment film is not necessary to the device without alignment films. The electric resistance of the device is sometimes decreased by the interaction of the alignment film and the composition. However, the phenomena are not caused because of the absence of the alignment film. This kind of effect caused by the polymer and the polar compound can be expected for a device having a mode such as TN, ECB, OCB, IPS, VA, FFS or FPA.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. A composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device with a polymer sustained alignment type. A composition having positive or negative dielectric anisotropy is used for a device without alignment films. Examples of the liquid crystal composition having positive dielectric anisotropy are disclosed in the following patent documents No. 1 to 4.